1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
To control the fixing process in an image forming apparatus, a halogen lamp heater has been used as a fixing heater, and the temperature of the fixing heater has been operated by on-off control thereof. To achieve finer temperature control, a control method has been known in the art which involves generating a drive voltage by suitably selecting half waves from an AC (alternating current) waveform and applying it to a halogen lamp heater.
In such control methods, the number of half waves selected from an AC waveform in a predetermined cycle (duty cycle) is suitably selected according to the amount of heat required, and the effective drive voltage applied to the halogen lamp heater varies depending on the number of half waves selected from the AC waveform.
A halogen lamp heater has a specific standard voltage at which so-called halogen cycle becomes the most effective. When the number of half waves selected from an AC waveform is so small that the applied effective drive voltage is lower than the standard voltage, a phenomenon of filament erosion (also known as chemical attack) occurs due to the decreased temperature of the filament (tungsten) of the halogen lamp heater.
To cope with the problem, a heater controller that has been known in the art is configured such that all halogen lamp heaters are operated (turned on) at the maximum output in every predetermined period even in a stand-by mode so that the halogen cycle circulates, and then after the filaments are heated to a predetermined temperature, the halogen lamp heaters are not turned off but a drive voltage is generated by suitably selecting half waves from an AC waveform and is applied to halogen lamp heaters. The heater controller thus prevents a break of the filaments and also reduces flickers (JP 2011-257604A).
However, the amount of heat required for forming an image varies depending on the type and size of sheet that is used as a recording medium. For example, when an image is formed on a small sheet, the amount of heat required is small, and the voltage is applied in such a pattern that is composed of a small number of half waves selected from an AC waveform in a predetermined cycle (low duty cycle). Therefore, a problem with the prior art is that when a drive voltage is applied continuously in such an application pattern having a low duty cycle for a long time, the life of a halogen lamp heater is decreased due to an occurrence of chemical attack.